Oil exists in the small pores and in the narrow fissures and interstices within the body of the reservoir rocks underneath the surface of the earth. A Residual Oil Zone (ROZ) is a petroleum deposit located beneath an existing primary petroleum production zone or associated with a geologic unit from which petroleum has migrated, migrated through or been flushed. ROZs typically cannot be economically developed using convention petroleum production engineering or secondary enhanced oil recovery (EOR) techniques such as water flooding.
Crude oil production in U.S. oil reservoirs takes place in three distinct phases: primary, then enhanced secondary, and finally tertiary recovery. Natural processes within geologic units that host reservoirs cause the oil to flow into traps that provide primary production, which depends upon multiphase hydrodynamics and the characteristics of the reservoir rock and the properties of the entrained gas, water, and hydrocarbon fluids. In many reservoirs, which are the part of a much large aquifer system, a natural flow of underground waters may be the drive force (aquifer drive) to push and displace oil into traps. ROZs are associated with the zones through which aquifer drives takes place.
As oil saturations decline in zones through which aquifer drive takes place, the mobility of the residual oil decreases as more of it is trapped by capillary forces by “snap-off” or in dead-end pores. Significant oil remains in the pores, but it can no longer be made to migrate toward proximal production wells nor towards primary traps. If the oil in the ROZ is unreactive at this point it is necessary to provide external energy to achieve oil recovery from the ROZ. Secondary recovery in primary production zones can be achieved by injecting gas (gas injection) and/or water (water flooding) to re-pressurize the reservoir and reestablish a drive mechanism to displace oil and drive it to the wellbore. Water flooding is a dominant means of secondary recovery and is implemented by injecting water into a set of wells while producing from the surrounding wells. When it becomes uneconomic to continue water flood operations tertiary recovery can begin by injecting CO2. Commonly if a water flood worked well in a reservoir tertiary recovery will also perform well, and data and information gathered during primary and secondary recovery can be used for design and operation of the CO2 EOR program.
A differentiating element of ROZs compared to the primary, secondary, and tertiary recovery from primary production zones is that development proceeds by directly using CO2 injection. All of the historical data and operational elements available from primary zone production are not available for ROZ production.
The use of CO2 injection for the recovery of petroleum from ROZs involves injection of compressed CO2 into the reservoir where it makes contact with oil, increasing oil mobility and increasing the amount of oil that is moved to production wells.
CO2 EOR involves reducing the interfacial tension between the oil and geologic matrix, changing the oil's viscosity, swelling the oil, and effectively releasing most of it from the geologic matrix pores. Some of the injected CO2 is exchanged for the displaced oil and water in the pores, and remains lodged in the formation via several mechanisms, including capillary, phase, solution, structural and stratigraphic trapping. These processes ultimately contribute to the permanent sequestration of the CO2.
Today, with much of the easy-to-produce oil recovered from U.S. oil fields using primary and secondary techniques, producers are now using a variety of tertiary, or enhanced oil recovery (EOR), techniques with the goal of producing 30 to 60 percent, or more, of the reservoir's original oil. United States reserves of oil associated with primary production zones is estimated to be 100 billion barrels. Estimates of oil reserves that are distinct to ROZs are early and likely to increase, but currently are in the range of 30 billion barrels in the Permian Basin alone.
A key element for the exploration, assessment and actual operation of a CO2 EOR program in an ROZ is the residual petroleum concentration and the chemical/physical state of that petroleum. Petroleum concentrations can be too low for exploitation (currently that lower concentration number is 12% of the available pore space). Current industrial practitioners of ROZ development estimate that the ultimate future magnitude of the petroleum resource in ROZs may be in the range of 50% to 100% of all known primary petroleum production.
The composition of oil and its effects on the physical chemistry of hydrogen sulfide (H2S) partitioning between the oil and water affect the residual oil saturation in an ROZ. H2S is a toxic, corrosive gas found in many oilfield production systems. While H2S can be indigenous to oil fields, it also can be generated within a reservoir by sulfate-reducing microbes as a result of injecting sulfate-containing water during water flood and also by consumption of sulfate from anhydrite or gypsum in the mineral matrix, causing reservoir souring. Residual oil saturation in an ROZ is significantly determined by the microbial activity of sulfate reducing microbes identified by Domain (Bacteria+Archaea+ekararyote fungi) and species which obtain energy by oxidizing organic compounds or molecular hydrogen (H2) while reducing sulfate (SO4−2) to hydrogen sulfide (H2S). It is important to evaluate specific metabolic pathways and the distinction of heterotrophic and autotrophic microbes which will affect the carbon balance and stoichiometry in the ROZ.
Microbes consume sulfate and hydrocarbons in the process, and the degree of hydrocarbon consumption is limited due to H2S inhibition, typically beginning at concentrations ranging from about 50 to about 400 mg/l. Without MSL conditions, over geologic time frames all of the oil in an ROZ would biodegrade. The MSL condition not only preserves the residual oil in place, but provides useful exploration, assessment, and operational tools (as previously described) as well.
When CO2 is used for enhanced oil recovery, a portion is retained in each usage cycle with ultimately some portion of it irretrievably retained (i.e., sequestered) through a combination of capillary, solution and physical trapping mechanisms.